Wave movements in oceans and large lakes is an important source of energy which can be utilized by harvesting energy from the waves by means of wave power assemblies, also called wave energy converters, which are positioned or anchored at locations with suitable wave conditions.
A number of different types of wave energy converters for harvesting and converting wave energy into electrical power are previously known. An example are so-called linear generators, which, by means of a rope or other connecting means, can convert vertical movement of a buoyant body caused by wave movements into a reciprocating movement of a generator coil or rotor of a linear generator that is anchored to the sea or lake bed. The reciprocating movement of the generator coil/rotor, in its turn, produces electrical current in the windings of an adjacent stator of the linear generator anchored to the bed.
Another previously known type of wave energy converter comprises a wave energy collecting buoyant body with an energy collection and conversion system, which may be placed on the sea bed. The buoyant body is connected to a winch via a winch cable. The winch and the winch cable connects the buoyant body to a reference body below the water surface, such as an anchor platform beneath the water surface, an anchor at the sea bed, or a different anchoring device. As the wave forces causes the buoyant body to move in the longitudinal direction of the winch cable, the winch is forced to rotate, whereupon the rotary motion of the winch axle produced can be converted into electricity by means of an energy conversion system.
Thus, in the two above-mentioned types of previously known wave energy converters, the upward and downward movements of the buoyant body resulting from wave movements produces a reciprocating movement in the longitudinal direction of a mooring line, cable or other connecting means, which connects the buoyant body to an anchor or other fixed anchoring point below the water surface. In order to obtain a high efficiency of these wave energy converters, it is essential that as large a fraction as possible of the motion of the buoyant body can be transformed into reciprocating motion in the longitudinal direction of the connecting means, which can be collected via the linear generator and winch, respectively, and be converted into electricity.
A completely different type of previously known wave energy converter is instead based on relative movement between, on the one hand, a buoyant body and a thereto attached so-called acceleration tube, and, on the other hand, a working piston reciprocable in the acceleration tube, wherein the relative movement is caused by wave movements in the body of water where the wave energy converter is anchored by means of one or several mooring lines for harvesting wave energy. The movement of the working piston can be used for driving for example a pump assembly, such as a double acting hydraulic pump or a hose pump, a hydraulic motor and/or a hydraulic turbine of an energy conversion system, which is disposed inside or adjacent to the buoyant body for producing electricity which can be transferred to an energy storage or electrical grid.
In the above-mentioned type of wave energy converter having an acceleration tube, it is thus inessential for the efficiency whether as large a fraction as possible of the motion of the buoyant body can be transformed into reciprocating motion in the longitudinal direction of the mooring line or not. As a matter of fact, such a wave energy converter having an acceleration tube can, in principle, operate without any mooring line at all, although this is not recommendable, both for safety reasons and since it is, of course, desirable to be capable of maintaining the wave energy converter at a desired anchoring position, where the wave conditions are as good as possible for wave energy harvesting.
The previously known mooring systems for wave energy converters having acceleration tubes have generally comprised fixing eyes, fixing cleats and/or shackles for mooring lines, wherein the mooring lines, in conjunction with anchors laid out, are intended to maintain the wave energy converter horizontally at a selected anchoring position, while the wave energy converter is still allowed to pivot vertically.
In practical trials it has been found that the previously used mooring systems of wave energy converters having acceleration tubes are associated with problems that may influence the durability, function, operational reliability and efficiency of such wave energy converters in a negative way. One problem with the previously known mooring systems of wave energy converters having acceleration tubes is that they often fail to maintain the wave energy converter within a selected, desired anchoring area during varying wind and current conditions and water levels, which may result in an inferior efficiency of the wave energy converter if it drifts away from a selected anchoring position with optimum wave conditions or, in the worst case, even result in breakdowns if the wave energy converter gets to close to a surf zone due to strong winds and/or variations in water level.
Another problem with the previously known mooring systems is that they do not keep the mooring lines taut when big waves or swells are passing the wave energy converter, which may cause a mooring line to make a knot onto itself, or make a loop around, and possibly damage the buoyant body of the wave energy converter when the mooring line gets slack in a deep wave trough, which may lead to operational failures, increased need of maintenance and reduced efficiency of the wave energy converter.
Yet another problem with the previously used mooring systems of wave energy converters having acceleration tubes is that their fastening devices for mooring lines on the buoyant body have a design which permits slippage between the mooring lines and the fastening devices, e.g. between a mooring line and an eye, a yoke, a fairlead and/or a hawse or shackle, as the buoyant body is thrown about by wave movements, which results in an increased need of maintenance work for replacing mooring lines, and which, in the worst possible case, can result in line failures and risk of damage to, or loss of the wave energy converter.
Additional problems solved by the present invention will become evident from the following description.